Turkey is buying Russian S-400 anti-aircraft missile systems from Russia, according to its president Tayyip Erdogan, surprising its NATO allies, the BBC News reported Tuesday.

Turkey has been developing stronger ties with Russia and souring on its relationship with the United States, which has supported the YPG Syrian Kurdish rebels during the Syrian civil war, the BBC News reported.

Erdogan's deal with Russia, for which he said a deposit has already been paid, is worth about $2.5 billion, the BBC News reported. Russian officials have said that its S-400 system has a range of 248 miles and can shoot down up to 80 targets simultaneously, aiming two missiles at each one.

Russia deployed the S-400 at its Syrian air force base near Latakia in December 2015, after Turkish military jets downed a Russian Su-24 warplane along the Syria-Turkey border, the BBC News said.

The incident muddied relations between Turkey and Russia, but Erdogan has reportedly made amends with Russian President Vladimir Putin, the BBC News noted.

"I can assure that all the decisions made for this contract strictly comply with our strategic interests," Vladimir Kozhin, an aide to Putin, told the Russian state news agency TASS, according to broadcaster Deutsche Welle. "In this regard, the reaction of some Western countries that are trying to put pressure on Turkey is completely understandable to us."

U.S. Sen. Ben Cardin, who sits on the Senate's foreign relations committee, suggested that the deal could violate sanctions against Russia while others see it as Russia's increasing influence in a NATO country, DW noted.

"It is a very significant development," Marc Pierini, former European Union diplomat and analyst at Carnegie Europe, told DW. "This is a missile defense system that is going to be hosted by the Turkish air force, and the Turkish air force has no experience of anti-missile systems, therefore it is going to come with a significant number of Russian advisers, trainers, and operators and so on. So at the top of the Turkish air force defense architecture, you're going to have Russians."